Fix: assoc types get workspace assignment (#376).

Analogous to topic types:
    - interactively created association types are assigned to the current workspace.
    - association types created through migrations are assigned to the default workspace ("DeepaMehta"), provided they belong to the "dm4" URI namespace. For other namespaces *no* workspace assignment is done automatically. In that case workspace assigment is up to the (3rd-party) developer.

BREAKING CHANGES

Public Workspaces API:
    - getWorkspaces() is renamed to getAssignedWorkspaces().    ### You must adapt your plugins
    - assignTypeToWorkspace() is new method. It assigns a type *and* all its view configuration topics to a workspace.

Close ticket 376.